Adulterous
Again it is important to note that Milton did not mean to defile all sex, especially between married people. Adulterous here means "sinful", or "lustful." It is then not the act, but the thought behind it that makes it sinful. Much time was spent by Milton in Book IV explaining how sex for procreation is a holy matter, the "true source / Of human offspring" (IV, 750-1). It was a particularly Puritan belief, but one that still holds sway among not only some Christian churches, but also the Mormon churches.